Caption: Scanning electron micrograph showing the backward slanting barbs on the terminal region of the spine of the cactus Opuntia aurantiaca. The spine is about 15mm long with barbs on the terminal 2mm. They slide in easily, withdraw painfully, or more likely, break off in the skin. Spines protect the plant from grazing animals, an important function in the arid regions where they grow. In some cases the spines & bristles (glochids) form a dense covering over the stem, sheilding it from sunlight. Spines develop from stem buds, which in cacti are paired; the lower one forms the spine, the upper one, flowers or branches. Magnificaion: X700 at 10x8 inch, x200 at 6x7cm size.